No it isn't, you can find studies that talk about how FSL requires an uncomfortable degree of tension applied to achieve the same elongation as achieved during an erection. For studies of trends it's useful to have a number, but FSL can be an inch or more out on erection length.

Stretched flaccid length can be greater or less than erect length in an individual guy. Most studies that have actually compared stretched flaccid length and erect length in samples of men have found that stretched flaccid length slightly underestimates erect length, but it does depend on how hard you stretch, and the difference in averages is typically much less than an inch.

No it isn't, you can find studies that talk about how FSL requires an uncomfortable degree of tension applied to achieve the same elongation as achieved during an erection. For studies of trends it's useful to have a number, but FSL can be an inch or more out on erection length.